A resin having excellent transparency is used for applications such as optical materials, illumination materials, signboards and decoration members. Fire retardancy may be required for these applications.
As a method for improving fire retardancy of a resin, known is a resin composition in which an inorganic compound such as basic magnesium carbonate is compounded in a resin (for example, refer to Patent Document 1). However, the above resin composition may not be suitable for use in a material which requires high transparency, such as optical materials because it usually has low transparency.
As other methods for improving fire retardancy, known are resin compositions comprising phosphoric acid ester or halogen-containing condensed phosphoric acid ester (for example, see Patent Documents 2, 3 and 4). However, phosphoric acid ester or halogen-containing condensed phosphoric acid ester comprised in these resin compositions may bleed on a surface during the course of usage to cause surface whitening and reduced fire retardancy. Further, resin compositions comprising halogen-containing condensed phosphoric acid ester have a risk of generating a halogen gas upon combustion, the halogen gas is toxic and metal-corrosive substance.
Further, proposed are copolymers in which phosphonate monomers or phosphate monomers are copolymerized with alkyl (meth)acrylate ester (see Patent Documents 5 and 6, Non-patent Documents 1 and 2). However, in order to improve the fire retardancy of these copolymers, the copolymerization ratio of phosphonate monomers or phosphate monomers needs to be increased. Since this results in a decreased glass transition temperature of a copolymer to be obtained, the copolymer will not be suitable for use at high temperature.